Due to the impact from diffraction of optical system, resolution obtained from conventional far-field microcopy method is limited. According to Abbe's diffraction limit theory, dimension of the light spot formed through focusing of the light beam by the microscope is indicated as
      Δ    ⁢                  ⁢    r    =      λ          2      ⁢      NA      by using the full width at half maximum; wherein, λ refers to the working wave length of the microscope; NA refers to the numerical aperture of the microscope used. Therefore, limiting resolution of conventional far-field optical microscope is normally restricted to about half wave length.
In recent years, scientific researchers have proposed numerous super-resolution microscopy methods in an attempt to break through restrictions of optical diffraction limit, and improve resolution ratio of the microscopy system.
For instance, the Stimulated Emission Depletion Microscopy (STED) makes use of the non-linear relationship between the fluorescence saturation and excited fluorescence excitation loss, restricts the area of excited radiation attenuation and reduces the size of the fluorescent light spot to obtain the luminous point below the diffraction limit, improve system resolution ratio, break through restriction on resolution by the diffraction limit of far-field microscopy, and achieve non-contact 3D imaging. Others include Structured Illumination Microscopy (SIM) and Stochastic Optical Reconstruction Microscopy (STORM), etc.
Despite of the fact that they all can achieve fluorescent super-resolution microscopy at the far field and obtain corresponding applications during practical tests, aforesaid several methods still have their own disadvantages. Among them, resolution for STED microscopy is determined by the light power consumed; therefore, the light power required in realization of high resolution is very strong, which is apt to incur bleaching to fluorescent molecules. Furthermore, STED microscopy system is relatively complicated, which is normally expensive. Despite of the fact that SIM microscopy has lower requirements for light power, it still requires raster scanning, which may result in relatively slower image formation and complicated image forming system. Image formation speed of STORM microscopy is also very slow, which is unlikely to be applied to real-time test of living cell at present.